This invention relates to an electrically actuated vehicle brake that utilizes electromagnets (EMs) to actuate vehicle brake shoes and more particularly to an improved electromagnet construction for such a brake. Such systems must be reliable and have a long life with a response that has low variability of any kind. The EM is attached to one end of a lever that is attached to a backing plate. There is a light spring force between the lever and the EM, thus putting the EM in contact with the face of the brake drum. The EM, when energized, forcibly drags against the face of a rotating brake drum and effects pivotal movement of the lever to actuate the brake shoes. The EM is mounted for limited movement relative to the lever so as to ride flat on the face of the brake drum disk during braking. When an electric current is passed through the coil, the side of the EM housing that faces the face of the brake drum disk is drawn against the rotating brake drum. The lever to which the EM is attached in turn expands the brake shoes into frictional engagement with the brake drum.
The amount of resultant braking is a function of the amount of electrical current supplied to the EM and the coefficient of friction between the EM and the brake drum disk. As the current increases, the magnetic force of the EM against the brake drum disk creates an increasing frictional drag. The brake shoe actuating arm moves arcuately (within its movement limits) against the arm springs. When the electrical current is decreased, the braking force is lessened. The brake shoe retraction springs operate to retract the brake shoes from engagement with the brake drum and also to return the brake shoe actuating arm to the brake release position. Since electric brakes rely on an electromagnet to convert the electrical energy supplied by a controller to mechanical energy, safety and reliability of the vehicle brakes depend on the low variability and the high repeatability, effectiveness, and reliability of the electromagnet.
EMs for actuating vehicle brakes have included cast, stamped, and sintered powder metal (PM) EM housings. In general, the EM housings have been cup-shaped and have provided an annular opening to receive a coil winding. Typically, after the coil is positioned within the annular opening, the housing opening is closed with a molding material and it is this visage that develops attractive and frictional drag.
Most of the currently available magnets in the industry use an epoxy-like material or an injection molding compound to encapsulate the magnet coil in the iron core of the EM and are filled flush to the active frictional face. This material comes in contact with the surface of the brake drum disk. As the material heats, it tends to change its form and can deposit residue on the brake drum. This residue, which is sometimes slippery, cohesive and/or adhesive, tends to cause the brakes to slip, then grab, then slip, and then grab. Some of the material used can also create very low friction and wear (such as in the case of nylon-like material) and can prevent the EM from readily wearing if it stands proud (i.e. prevents the metal from touching). Due to the oftentimes high thermal expansion coefficient and/or high tendency to expand with moisture, this can be a problem as desired frictional drag is uncertain and often greatly reduced. The delayed functional contact of the EM core with the opposing moving metal surface is highly undesirable and dangerous. In both cases the plastic material that is used does not keep the metal-to-metal surfaces from galling and/or does not exhibit the desired frictional drag characteristics.
Another approach that has been employed is discussed in U.S. Pat. No. 3,668,445 to Grove. Grove uses a frictional insert that is supposed to have a lower wear rate than the PM and is supposed to supply the frictional drag of the unit by way of it standing proud. Grove's explanation is that the primary frictional drag comes not from the metal-to-metal interface but from the insert and the brake drum disk. However, Grove's insert material can carry little force due to its low modulus of elasticity. Thus, approximately 99% of the frictional drag comes from the metal-to-metal contact.
Grove U.S. Pat. No. 3,760,909 discloses grooves for the purpose or removing surface dust. With the attractive force of the EM in the 200-lb. range and considering the surface speeds, as well as the area of the brake drum disk as compared to the area of the EM, this is not viewed as a primary problem.
Pressed sintered PM housings have been widely used for electromagnets due to the low cost of manufacturing relative to other methods. Another prime advantage is that very low-carbon high-purity annealed iron can be used that has highly desirable magnetic properties such as having high magnetic saturation capabilities. The disadvantage of the current powdered metal EMs is that they degrade from moisture infiltration. Environmental moisture infiltration can readily occur in powdered metal electromagnets even as they are stored. Moisture infiltration of the powdered metal causes internal corrosion of the powdered metal causing it to have a lower level of magnetic saturation. This reduced magnetic saturation level reduces the drag force that the electromagnet can apply to a drum brake. Degradation of the powdered metal electromagnets due to moisture infiltration has been observed to cause high variability by reducing the drag force of commercially available EMs. As the powder metal corrodes, maximum magnetic saturation level is reduced. The impact can be as high as 65% reduction in the effectiveness of an EM prior to or after being installed in an electric brake. Use of copper infusion, and other like approaches, decreases the allowed magnetic saturation an impractical amount. Use of nonporous coatings cannot exist on the wearing metal-to-metal contact that is required at the EM to drum disk interface. Therefore, moisture can still enter the EM on that surface. Commercially available powdered metal electromagnets that have not yet degraded on the storage shelf can readily degrade in the field upon exposure to moisture. Typically, such EMs in use have had high variability from unit to unit in operating the brake mechanism.
The current commercially available EMs suffer from premature local magnetic saturation effects within their magnetic circuits that limit the magnetic field that can be produced. This effect is due to variable magnetic cross section in the core structures. The result is that they use more excitation current, larger copper, and more turns to get the magnetic force that is required. The cost of producing such units and the total current for operating a braking system is great. The required power to operate a system using these devices is very high. The wiring installed system excitation wiring resistance for such a system has to be lower due to the higher required operating currents, thus increasing the cost of installation by requiring heavier copper wiring. If smaller copper is used, then the sensitivity of various parts the installation becomes a greater concern in maintaining equal braking responses for the various wheels because of the variations in the excitation circuit for the various axles.
Some units that are marketed will burn out due to high energy dissipation when on the work bench. When in contact with the brake drum disk, the unit experiences a large protective heat sink; however, when in the process of braking a vehicle, the frictional drag of the EM can produce heat on the order of a thousand watts. Thus the magnetic core heat sink of the coil is at a high temperature that can be on the order of 375 degrees Fahrenheit. The coils and potting of commercial units do not prevent this problem.